There is an increasing interest in the use of carbon nanotubes in the electronic field, and in general, in information technologies. This interest is particularly due to the electrical properties of carbon nanotubes, which substantially depend on their structure and geometry. Carbon nanotubes (CNTs) substantially include cylindrical structures of atoms of carbon arranged in a hexagonal configuration, and having a high length-diameter ratio (e.g., diameters on the order of several atoms and lengths up to several microns). Nanotubes can be both single-wall (SWNT, Single-Wall carbon Nano Tube), and multi-wall (MWNT, Multi-Wall carbon Nano Tube) formed by two or more coaxial structures of SWNT. On the basis of the diameter and the chirality, i.e., the value of the angle of the hexagonal structure which composes them, carbon nanotubes can show a metallic or semi-conductive behaviour. In particular, the natural use of metallic nanotubes is the fabrication of nanowires, which essentially conduct current on a surface and provide small, low resistivity interconnections that conduct high density current. As semiconductors, they can instead be integrated as part of a transistor.
To selectively exploit the electrical properties of the typology of carbon nanotubes, several technologies have been devised which enable separation of the semiconductor nanotubes from the metallic ones after they have been developed. The nanotubes can then be further separated by diameter or chirality—this enables nanotube powder/solution sources with different specific electrical properties. In particular, the possibility of controlling the chirality stands for the possibility of making a real new generation of electronic devices comprising circuits only formed by SWNTs, or alternatively by MWNTS, wherein semiconductor nanotubes play the role of active elements (e.g. transistors), while metallic nanotubes play the role of connectors.
CNT arrays are currently grown from pairs of patterned iron catalyst strips. The problems with this technique are that the CNTs produced are 2:1 semiconducting to metallic and that the diameter (bandgap) of the semiconducting CNTs cannot be controlled. Bandgap control is crucial because the CNT bandgap is what sets the injection barrier between the source contact and the CNT. Since the iron catalyst grown CNTs have a bandgap range of 0.4-1.3 eV, not all of the tubes can be contacted equally well with a single source contact material. The work function of the contact metal is typically high; however, the Fermi level of CNTs is about 4.6 eV. This means that the large bandgap tubes are difficult to contact and contribute very little to the total on current of a CNT field effect transistor (FET).
On the other hand, the small bandgap, semi-metallic tubes are easy to contact, but don't turn off. These, along with fully metallic CNTs, increase power consumption in mixer devices and degrade the on/off ratio, which means that iron catalyst FETs would perform poorly in an amplifier. This variation in CNT bandgap is also consistent with the observed variation in the minimum voltage (Vmin) values of CNT array FETs. Furthermore, empirical modeling has shown that strong Vmin variation results in low on/off ratios, even in arrays containing all semiconducting carbon nanotubes.